A known on-board evaporative emission control system for an automotive vehicle comprises a vapor collection canister that collects volatile fuel vapors generated in the headspace of the fuel tank by the volatilization of liquid fuel in the tank and a purge valve for periodically purging fuel vapors to an intake manifold of the engine. A known type of purge valve, sometimes called a canister purge solenoid (or CPS) valve, comprises a solenoid actuator that is under the control of a microprocessor-based engine management system, sometimes referred to by various names, such as an engine management computer or an engine electronic control unit.
During conditions conducive to purging, evaporative emission space that is cooperatively defined primarily by the tank headspace and the canister is purged to the engine intake manifold through the canister purge valve. A CPS-type valve is opened by a signal from the engine management computer in an amount that allows intake manifold vacuum to draw fuel vapors that are present in the tank headspace and/or stored in the canister for entrainment with combustible mixture passing into the engine's combustion chamber space at a rate consistent with engine operation so as to provide both acceptable vehicle driveability and an acceptable level of exhaust emissions.
Certain governmental regulations require that certain automotive vehicles powered by internal combustion engines which operate on volatile fuels such as gasoline, have evaporative emission control systems equipped with an on-board diagnostic capability for determining if a leak is present in the evaporative emission space. It has heretofore been proposed to make such a determination by temporarily creating a pressure condition in the evaporative emission space which is substantially different from the ambient atmospheric pressure, and then watching for a change in that substantially different pressure which is indicative of a leak.
It is believed fair to say that there are two basic types of diagnostic systems and methods for determining integrity of an evaporative emission space against leakage.
Commonly owned U.S. Pat. No. 5,146,902 "Positive Pressure Canister Purge System Integrity Confirmation" discloses one type: namely, a system and method for making a leakage determination by pressurizing the evaporative emission space to a certain positive pressure therein (the word "positive" meaning relative to ambient atmospheric pressure) and then watching for a drop in positive pressure indicative of a leak. Other positive pressure type systems are disclosed in commonly owned U.S. Pat. Nos. 5,383,437; and 5,474,050.
The other of the two general types of systems for making a leakage determination does so by creating in the evaporative emission space a certain negative pressure (the word "negative" meaning relative to ambient atmospheric pressure so as to denote vacuum) and then watching for a loss of vacuum indicative of a leak. A known procedure employed by this latter type of system in connection with a diagnostic test comprises utilizing engine manifold vacuum to create vacuum in the evaporative emission space. Because that space may, at certain non-test times, be vented through the canister to allow vapors to be efficiently purged when the CPS valve is opened for purging fuel vapors from the tank headspace and canister, it is known to communicate the canister vent port to atmosphere through a vent valve that is open when vapors are being purged to the engine, but that closes preparatory to a diagnostic test so that a desired test vacuum can be drawn in the evaporative emission space for the test. Once a desired vacuum has been drawn, the purge valve is closed, and leakage appears as a loss of vacuum during the length of the test time after the purge valve has closed.
In order for an engine management computer to ascertain when a desired vacuum has been drawn so that it can command the purge valve to close, and for loss of vacuum to thereafter be detected, it is known to employ an electric sensor, or transducer, that measures negative pressure, i.e. vacuum, in the evaporative emission space by supplying a measurement signal to the engine management computer. It is known to mount such a sensor on the vehicle's fuel tank where it will be exposed to the tank headspace. For example, commonly owned U.S. Pat. No. 5,267,470 discloses a pressure sensor mounting in conjunction with a fuel tank roll-over valve.